Embodiments of the present invention relate to methods and systems for measuring A pore pressure of a subsurface formation surrounding a wellbore penetrating the subsurface formation and, more specifically but not by way of limitation, the methods and systems of such embodiments provide for measuring the pore pressure so as to remove/reduce the interfering effects and/or influences of the wellbore on the pressure being measured. More particularly, but not by way of limitation, in an embodiment of the present invention a pressure sensor and a temperature sensor may be disposed in a channel extending from the wellbore into the subsurface formation, the sensors periodically measure the pressure and temperature of the formation and these measurements are communicated to a processor that processes the periodic measurements of the pressure and temperature in the formation to determine a pore pressure of the formation that is free or substantially free from wellbore influences.
During the production of fluids such as hydrocarbons and/or gas from an underground reservoir, it may be desirable to determine the development and behaviour of the reservoir. Such reservoir determinations may allow production from the reservoir to be controlled and optimized and may also provide for determining how changes to the operation of the wellbore affect or may affect the reservoir. Formation pressure measurement is one measurement that may be made on a formation and used to provide for the management of the reservoir.
When a well is first drilled, it may be relatively easy to accurately measure formation pressure. For example, a probe may be positioned in contact with a borehole wall and used to sense the pressure of fluids or the like in the formation. Such measurement of formation pressure may be made by means of a tool that may be lowered into the wellbore via a wireline cable and pressure measurements may be logged through the well with the tool and cable being removed from the wellbore when measurements are completed. Because such formation-pressure-logging tools may be relatively large and expensive, the tools are not generally left in the wellbore for overly long periods of time.
In a normal wellbore drilling process, the step of completion of the wellbore may be realized by installing a liner or casing into the wellbore. The casing may be made of steel and may be fixed to the wall of the wellbore by a cement that may be disposed in an annulus between the outer surface of the casing and the borehole wall. The casing or liner may provide a physical support to the wellbore to prevent the wellbore collapsing or becoming eroded by flowing fluids. The completion of the wellbore prevents access being made to the formation from the wellbore. As such, one completed, it may be difficult to obtain accurate formation pressure measurements.
Various approaches have been proposed to enable measurements to be made on formations after a well has been completed in the manner described above. These approaches provide for positioning sensors in the formation, but contain many limitations, including but not limited to operation of the sensors, data collection and wellbore influences on data being measured.
U.S. Pat. No. 6,234,257 and U.S. Pat. No. 6,070,662 describe a system and method in which a sensor is disposed inside a shell, which is forced into the formation. This forcing of the shell into the formation is achieved by the use of an explosive charge that is fired while the well is being drilled. According to the reference, the sensor can then be interrogated for an extended period after the drilling is finished by means of an antenna which can communicate through an aperture provided in the casing.
SPE 72371 describes a tool (the CHDT tool of Schlumberger), which may be used with a pressure to provide for pressure testing of a formation after completion of a well. As described in the reference, the tool may be used to drill a hole through the casing and cement into the formation and a probe/sensor may be placed in the wellbore and over the hole to sense the formation pressure and take samples of any formation fluid, if required. Once the measurement is complete, a plug or rivet is placed in the hole in the casing, sealed and pressure tested to confirm the integrity of the casing.
Installation of permanent sensors on the outside of the casing of the wellbore may allow for long term monitoring of formation pressure. However, since cement associated with the casing is usually impermeable, such monitoring would necessitate to some means of fluid communication between the formation and the sensor in order that pressure can be measured. One proposal has been to mount the sensor in a chamber on the outside of the casing that also carried an explosive charge. After installation and cementing, the charge is fired to provide a communication path into the formation. This approach is not preferred in many cases since it requires the use of explosive charges which brings with it safety considerations and extensive complexity for controlling the firing of the charge. The damage caused by the charge might be sufficient to damage the sensor too. Another potential problem is that since the perforation tunnel is not open to the well, fluid does not flow through the perforation and allow cleaning of residues. Therefore there is no way to ensure that there is good fluid communication between the formation and the sensor. Since the charge is mounted on the outside of the casing, it may be necessary to use a smaller casing size than normal to fit into the borehole. Further details of this approach can be found in U.S. Pat. No. 5,467,823.
Furthermore, patent publication Nos. US 2005/0217848, WO 2006/000438 and WO 2006/005555 disclose sensors and methods for disposing such sensors behind the casing of the completed wellbore. The positioning methods including drilling through the casing and cement into the formation surrounding the well so as to create a fluid communication path and sealing the hole drilled in the casing.
While the above mentioned patents and patent applications disclose concepts for obtaining formation pressure in a subsurface formation penetrated by a wellbore they do not identify the problems associated with wellbore influences on the formation pressure or disclose methods and systems for accurately measuring such a formation pressure free of such wellbore influences. Nor do the patents and patents applications consider low permeability formations or disclose how to make an accurate formation pressure measurement in a low permeability formation, such as shale or the like.